regulator: pwm-regulator: Fix 'unused-variable' warning

drivers/regulator/pwm-regulator.c:
  In function 'pwm_regulator_init_continuous':
drivers/regulator/pwm-regulator.c:202:22:
  warning: unused variable 'np' [-Wunused-variable]
     struct device_node *np = pdev->dev.of_node;
              ^
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>